1. Field of the Invention
The present invention relates to electrical apparatus, and more particularly to circuit interrupters having current limiting capability.
2. Description of the Prior Art
Circuit interrupters are widely used to provide protection for electrical distribution systems against damage caused by overload, or fault, current conditions. As the capacity of power sources increased, it was necessary to provide increased interrupting capability for circuit breakers to properly protect the electrical distribution system. In order to provide this protection in a more economical manner, current limiting circuit interrupters were developed to limit the amount of fault current to a level below that which the source was capable of supplying.
One type of current limiting circuit interrupter employs a high-speed mechanism to rapidly separate the contacts upon occurrence of a fault condition to draw an arc between the contacts, allowing the voltage drop across the arc to limit the current flow. The electrodynamic force produced by the current flow through the circuit interrupter is used to rapidly drive the contacts apart and force the arc into an extinguishing device. The standard trip mechanism then actuates to maintain the contacts in the open circuit position.
All circuit breakers require a certain amount of contact closing force to reduce resistance between the contacts and, consequently, the amount of resistance heating generated during normal closed circuit conditions. This contact force is most commonly obtained by means of extension or compression springs attached to the contact arm. The higher the current rating of the circuit breaker, the greater the required contact force.
In a current limiting circuit breaker, however, the contact arms separate independently of other portions of the operating mechanism to produce the current limiting action and, in the process, stretch or compress the springs further from their normal positions. The resisting force supplied by these springs during current limiting operation thus significantly reduces the acceleration of the contact arms and the degree of current limiting, especially with higher circuit breaker ratings. It is therefore desirable that the contact spring force be eliminated or kept to a minimum in order to produce maximum acceleration of the contact arm during current limiting, yet provide sufficient contact closing force during normal closed circuit conditions to reduce resistance heating of the circuit breaker contacts.
A current limiting circuit breaker supplying this contact spring force action must also provide the latching function needed to maintain the contacts in an open position during current limiting operation until the trip device can operate. Various methods have been employed to provide this latching function; however, for a variety of reasons they have not proven entirely satisfactory. It would be desirable, therefore, to provide a current limiting circuit interrupter having low-resistance contacts during closed circuit conditions which will rapidly separate upon occurrence of severe overload current conditions to provide current limiting operation, and a mechanism latching the contacts in the open circuit position until the normal trip mechanism can operate. In order to reduce costs and increase reliability, it is desirable that such a mechanism have a minimum of parts.